This invention relates to the protection of trialkali metal nitrilotriacetate in an aqueous solution containing active chlorine.
The property possessed by some materials of improving the detergency levels of soaps and synthetic detergents and the use of such materials in detergent compositions is known. Such cleaning boosters are called "builders" and such builders permit the attainment of better cleaning performance than is possible when so-called unbuilt compositions are used. The behavior and mechanisms by which builders perform their function are not completely understood. It is known that good builders must be able to sequester most of the free calcium and/or magnesium ions in the wash water since these ions are detrimental to the detergency process. However, it is difficult to predict which class of compounds possess useful combinations of builder properties and which compounds do not because of the complex nature of detergency and the countless factors which contribute to overall performance.
The alkali metal salts of nitrilotriacetate have been found to be highly efficient cleaning and detergency builders and these compounds, particularly trisodium nitrilotriacetate, have been employed with good results in cleaning and detergent formulations. Indeed, millions of pounds of trisodium nitrilotriacetate are used each year in cleaning formulations because of its superior builder qualities. However, when a chlorine-containing bleach, such as sodium hypochlorite, is added to the aqueous solution containing a detergent formulation using a trialkali metal nitrilotriacetate as a builder, the chlorine can react rapidly with free nitrilotriacetate anion that has not already complexed calcium and/or magnesium ions, resulting in a loss not only of the beneficial effects of the excess quantity of nitrilotriacetate as a detergency booster, but also with the concurrent loss of active chlorine for use as a bleach.
U.S. Pat. No. 4,148,742 issued Apr. 10, 1979 discloses that the alkali metal salts of imidobisulfuric acid, particularly the trisodium salt, are effective in heavy duty laundry detergent compositions by reducing the damaging effects of chlorine-containing bleaches on the fabric while minimizing the loss in effectiveness of the bleach to remove stains from soiled fabrics. Further, Japanese Pat. publication No. 153,537 dated Dec. 23, 1975 discloses detergent compositions containing an anionic surfactant, a trialkali metal imidodisulfate, and trisodium nitrilotriacetate or sodium citrate to improve the builder action of the imidodisulfate, allowing lower concentrations of the latter to be used. However, it has been found that in hard water containing 150 ppm calcium hardness or greater, the trialkali metal imidodisulfates are only about 20 percent as effective as trisodium nitrilotriacetate as a builder.
Despite the advantages taught in these and other references in the prior art, it has now been surprisingly found that the trialkali metal nitrilotriacetates, when in aqueous solution also containing active chlorine, can be protected from damage by the chlorine by the process of the present invention. Thus, the builder properties of the non-effective trialkali metal nitrilotriacetate are maintained at a high level. Even though trialkali metal imidodisulfates have been disclosed as useful to reduce the damaging effects of chlorine-containing bleaches on the fabric while minimizing the loss in effectiveness of the bleach in removing stains from the soiled fabric, it was surprisingly found that the alkali metal salts of imidobisulfates would also protect the nitrogen-containing nitrilotriacetate from damage by the active chlorine. According to the method of the present invention, it has now been found that the builder properties of the trialkali metal nitrilotriacetates can be protected in an aqueous solution containing active chlorine.